1. Field of the Invention
The present invention relates generally to electron beam exposing apparatuses, and more specifically, to stencil masks.
2. Description of the Background Art
Lithography is one step in a manufacturing process of a semiconductor device. By a lithography technique, a thin film such as a silicon oxide film is formed on a semiconductor substrate, resist is formed thereon, the resist is exposed and formed into a prescribed shape, and the thin film is etched using the resist as mask to form the thin film into a prescribed shape.
Electron beam, whose wavelength is shorter than X ray permits fine treatment. A conventional lithography technique utilizing an electron beam will be described.
As illustrated in FIG. 9, a silicon oxide film 3 is formed on a silicon substrate 1, and then resist 5 is formed thereon. As illustrated in FIG. 10, resist 5 is exposed into a prescribed shape using an electron beam. 5a denotes the exposed part, while 5b denotes the non-exposed part.
Resist 5 is developed and exposed part 5a is removed away as shown in FIG. 11. As shown in FIG. 12, silicon oxide film 3 is etched away utilizing the non-exposed part 5b of resist 5 as mask. The non-exposed part 5b of resist 5 is removed away as shown in FIG. 13. Thus, the conventional lithography utilizing electron beam is complete.
FIG. 14 is a view schematically showing a conventional electron beam exposing apparatus. The structure of the conventional electron beam exposing apparatus will be described. Disposed in a chamber 7 are, from top to bottom, an electron gun 31, an illumination lens 33, a beam forming aperture 35, a beam forming deflector 37a, a stencil mask 41, a positioning deflector 40, and an objective lens 43. An electron beam 39 is emitted from electron gun 31. Illumination lens 33 adjusts the cross sectional area of the electron beam 39 so that the electron beam 39 covers the opening portion 36 of beam forming aperture 35.
Beam forming aperture 35 at its opening portion 36 forms the cross sectional shape of the electron beam 39 into a square. The beam forming deflector is arranged as shown in FIG. 15 when viewed from above, and a beam forming deflector 37b is not illustrated in FIG. 14. Beam forming defector 37a deflects the electron beam 39 in an A direction, while beam forming deflector 37b deflects the electron beam 39 in a B direction.
Stencil mask 41 has a square opening 55 at its center as illustrated in FIG. 16. The cross sectional shape of the electron beam 39 passing through opening 55 is changed by controlling the position of the electron beam 39 by beam forming deflectors 37a and 37b.
Referring to FIG. 14, the electron beam 39 which has passed through opening 55 is deflected by positioning deflector 40. The electron beam 39 is directed at a prescribed position on a silicon wafer 45, and its adjustment is made by positioning deflector 40. The electron beam 39 having passed through positioning deflector 40 proceeds toward objective lens 43. The electron beam 39 is converged, and the converged electron beam 39 is directed at silicon wafer 45.
A description will be provided on a method of exposing a prescribed pattern on resist utilizing the electron beam exposing apparatus shown in FIG. 14. A thin film (not shown) is formed on silicon wafer 45, and resist 57 is formed thereon. When resist 57 is selectively exposed in the pattern denoted with 59, the pattern 59 is divided into 59a, 59b, and 59c shown in FIG. 18 for exposure. More specifically, the position of the electron beam 39 is controlled by beam forming deflectors 37a and 37b shown in FIG. 15 so that resist 57 is selectively exposed in the pattern denoted with 59a, and the cross sectional shape of the electron beam 39 passing through opening 55 shown in FIG. 16 is made into the shape of the pattern 59a. Similarly, resist 57 is selectively exposed in the patterns 59b and 59c.
If the electron beam 39 continuous to be directed at stencil mask 41 shown in FIG. 16, stencil mask 41 increases in temperature and is expanded. Accordingly, the shape of opening 55 changes. Meanwhile, if irradiation of the electron beam 39 is stopped, the temperature of stencil mask 41 decreases, thereby contracting stencil mask 41. Accordingly, the shape of opening 55 changes. More specifically, the shape of opening 55 is not always constant.
As illustrated in FIG. 19, it is assumed that stencil mask 41 is expanded to take a shape delineated by 41a, and opening 55 takes a shape illustrated with 55a. When the position of the electron beam 39 is controlled using opening 55 as a reference and the opening 55 takes a form illustrated by 55a, the cross sectional form of the electron beam 39 passing through the opening is different between 55 and 55a, and therefore the resist can not be exposed in a desired pattern. Furthermore, the position of exposure pattern changes.
In order to solve the above-stated problem, a material having a low coefficient of linear thermal expansion such as molybdenum is used for stencil mask 41. But the use of the material having a low coefficient of linear thermal expansion for stencil mask 41 still can not prevent the form of opening 55 from changing because of the electron beam 39 heating stencil mask 41.
A technique of keeping the form of opening of a stencil mask constant is disclosed in, for example, Japanese Patent Laying-Open No. 62-272529. FIG. 20 is a perspective view showing the stencil mask disclosed in the above-stated document. An opening 65 is provided in the center of a stencil mask 63. A heater 67 formed of a nichrome wire or the like is formed inside stencil mask 63.
The temperature of stencil mask 63 is kept at a prescribed value by the use of heater 67 thereby keeping the form of opening 65 constant. More specifically, if the temperature of stencil mask 63 is below the prescribed value, the amount of heating stencil mask 63 by heater 67 is increased, thereby returning the temperature of stencil mask 63 to the prescribed value. Meanwhile, if the temperature of stencil mask 63 is above the prescribed value, the amount of heating stencil mask 63 by heater 67 is decreased, thereby returning the temperature of stencil mask 63 to the prescribed value.
Since current is passed through heater 67 to heat stencil mask 63, a magnetic field is generated around heater 67. This affects the electron beam, the cross sectional form of the electron beam passing through opening 65 does not take a desired form and sometimes the resist can not be exposed in a desired form. Furthermore, in some cases exposure can not be made at a prescribed position.